The invention relates to an arrangement for fastening and wiring of a plurality of electrical units, particularly in a switchgear cabinet, which has at least one mounting crosspiece accommodating the electrical units on its mounting side and which can be fastened to at least two vertical bearing rails arranged behind, fastening means being provided for fastening the electrical units on the mounting crosspiece. Arrangements of this type for switchgear cabinets are known, for example, from the "LSC Wiring System" catalog of the firm of Friedrich Luetze GmbH & Co. The known mounting crosspieces have an essentially U-shaped cross-section and are connected at the edge with a frame construction by means of U-shaped mounting clamps. Due to the folded U-leg of the mounting crosspieces, the space behind these mounting crosspieces is difficult to access and is not available as wiring space. If needed mounting depth is provided, there are hardly any problems in the case of use in large switchgear cabinets. However, the switchgear cabinets are becoming ever smaller, because of ever smaller electrical units and progressive integration so that the role of the structural depth is becoming increasingly important. For this reason, one object of the present invention consists in improving an arrangement of the type mentioned at the outset so that a considerably smaller structural depth is achievable for use in very small switchgear cabinets as well. This object is achieved by the invention in that the mounting crosspiece shaped like a flat strip has, on its rear side opposite the mounting side and extending over its length, an integrally formed holding rib with a mounting groove open toward the mounting side, in that the mounting groove is designed to receive the heads of mounting screws for fastening the mounting crosspiece to the bearing rails by means of the holding rib, and in that at least one of the two opposed longitudinal edges of the mounting crosspiece shaped like a flat strip is provided with a wiring comb. The invented mounting crosspieces can be fastened without mounting clamps to two bearing rails which achieves not only a decrease in structural depth but also a reduction in mounting parts. Because of the design of the mounting crosspiece with the shape of a flat strip and without folded areas or legs, the space behind the mounting crosspiece is fully utilizable as wiring space except for the minor amount of space needed for the holding ribs, which is especially important and advantageous for very flat designs. The wiring comb arranged on at least one of the longitudinal edges does not represent any noticeable impairment of the access to the wiring space either. Mounting can thus be simpler and faster, which means a considerable savings in costs. The mounting crosspieces can extend up to the side walls of the switchgear cabinet since no mounting manipulations are to be carried out at the edge areas but rather only in the area of the inwardly shifted bearing rails. The arrangement of the bearing rails, particularly with respect to the interval between them, can be selected within wide ranges, so that there will be no problems with adjustment to local conditions. Advantageous further developments and improvements of the arrangement indicated in claim 1 are possible by applying the measures given in the sub-claims. It will be suitable for the arrangement of the holding rib to be substantially central and the holding rib will preferably have a substantially U-shaped cross-section. The central arrangement automatically provides for a symmetrical distribution of the available wiring space. Advantageously, both longitudinal edges of the mounting crosspiece are designed for plugging or locking in one wiring comb each, and both longitudinal edges of the mounting crosspiece are preferably designed as receptacles for plugging or locking in wiring combs. Simple plugging and locking in place make it possible to have rapid and simple snap mounting, and the wiring combs can be arranged as desired on only a few mounting crosspieces and there on both the longitudinal edges or only on one. Simple snap mounting also makes it possible to make rapid changes. The wiring space can be made as desired to meet requirements through the provision of spacers with recesses for the mounting screws between the holding rib and the bearing rails. At the same time, these spacers can serve to establish the right angle between the mounting crosspieces and the bearing rails and prevent a change in angle in that the spacers have a substantially square cross-section and they can be mounted in a torsionally stiff way by means of a groove-like depression on the side of the holding rib and by means of a groove-like depression on the side of the bearing rails. Preferably, the groove-like depressions will be formed between two bead-like projections extending in the longitudinal direction, that is, on both the holding rib and the bearing rails. The right angle will thereby be permanently fixed automatically when screws are tightened. It will be advantageous for the bearing rails to have at least one mounting groove that is open toward the holding rib and guides the sliding nuts cooperating with the mounting screws for longitudinal displacement. This makes it possible, with a slight loosening of the mounting screws, to shift the mounting crosspieces anywhere along the bearing rails and fix them rapidly and simply in the desired position. In addition, the bearing rails can also have a further mounting groove that is open toward the opposite side and guides the sliding nuts cooperating with further mounting screws for mounting the bearing rails on a cabinet wall or on holding means for longitudinal displacement. Preferably, holding strips that can be connected with the staybolts otherwise present in switchgear cabinets for mounting in such cabinets will be provided, the holding strips having slots for connection with the further mounting groove of the bearing rails. Here too, the position of the bearing rails can thus be freely selected within wide ranges and fixed by tightening the further mounting screws. This ensures a variable installation in switchgear cabinets with different dimensions. Sliding nuts are provided as fastening means for fastening the electrical units or the like on the mounting crosspieces, these nuts being longitudinally displaceable in guides in the mounting groove of the holding rib and spaced away from the base of the mounting groove such that the heads of the mounting screws are arranged outside the path of motion of the sliding nuts. This makes it possible to displace electrical units, for example, and to position them by means of sliding nuts without the heads of the mounting screws having any effect on free mobility. Alternative fastening means for fastening the electrical units can be created to advantage by providing the mounting crosspiece on the mounting side with two integrally formed hook-like strips for forming a cover rail or a C-rail. In particular, the two strips in this case are arranged symmetrically on both sides of the mounting groove. In this way, conventional cover rails, for example, can be formed simply with the invented arrangement. A number of parallel mounting crosspieces can be arranged in an advantageous way on the two bearing rails in particular. Mounting crosspieces having dimensions and/or fastening means designs that are different can also be placed in this case on the two bearing rails in any arrangement desired. An example embodiment of the invention is depicted in the drawing and explained in more detail in the description following.